The present invention relates generally to differential amplifiers. More specifically, a differential amplifier that has a wide range of operating frequencies is disclosed.
Differential amplifiers are often used in applications that require low noise amplification. FIG. 1 is a circuit diagram of a typical differential amplifier. Transistors 100 and 102 are connected to load resistors 104 and 106, respectively. The load resistors are connected to a voltage source, Vcc, at 116. Two input signals are applied to the gate terminals 110 and 112 of transistors 100 and 102. The output is taken between the drain terminals 118 and 120 of the transistors. Ideally, the output represents the difference between the two inputs.
Transistor 108 is a current source for transistors 100 and 102. The sources of transistors 100 and 102 are connected together with the drain of transistor 108, at node 114. Because the circuit is symmetrical, for AC input signals that are opposite of each other, the resulting signals at the sources of the transistors cancel each other at node 114. Thus, node 114 appears as AC ground for differential inputs.
The amplifier sometimes receives common mode inputs at its input interfaces. Common mode input signals are signals that are substantially the same. They are often caused by noise and are undesirable. The common mode signals are rejected when the common mode gain is small. To achieve small common mode gain, the impedance between node 114 and ground should be relatively high. In practice, however, the common mode gain is increased at higher operating frequencies due to parasitic capacitance, and the amplifier""s operating frequency range is reduced.
Ideally, the headroom consumed by transistor 108 should be as low as possible, thereby allowing greater output range for transistors 100 and 102. For a given current, a larger transistor can be used to achieve lower headroom. However, a larger transistor generally has larger parasitic capacitance associated with it, and would increase the common mode gain at higher frequencies and reduce the amplifier""s operating range. It would be useful to have a differential amplifier design that has low common mode gain at high frequencies, large operating frequency range and better headroom for the output transistors.